1. Field of the Invention
The present invention relates to a semiconductor optical element, an optical module and a method of manufacturing the semiconductor optical element.
2. Background Art
In the conventional optical communication, a communication system called a wavelength division multiplexing communication system is used. In wavelength division multiplexing communication systems, one optical fiber cable simultaneously accommodates optical signals with a plurality of different wavelengths.
In recent years, in wavelength division multiplexing communication systems, the difference between the oscillation wavelength of the shortest-wavelength laser element and the oscillation wavelength of the longest-wavelength laser element has become larger. More specifically, for example, this oscillation wavelength difference is as large as 15 nm in some communication standard.
As an optical module used for wavelength division multiplexing communication, an optical module having a plurality of integrated optical modulation elements combined in one package is known. A typical one of integrated optical modulation elements is an element formed by growing a laser element portion and an electric field absorption type of optical modulator portion provided side by side on a semiconductor substrate.
An optical module formed by arranging a plurality of integrated optical modulation elements on one semiconductor substrate, for example, as disclosed in Japanese Patent Laid-Open Publication No. 10-117040 in conjunction with FIG. 4 or the like is conventionally known. A wavelength division multiplexing communication system needs a plurality of different light sources and, therefore, has a plurality of laser element portions with different oscillation wavelengths integrated therein. Optical modulation can be performed on laser lights of different wavelengths from the laser element portions by using electric-field-absorption-type optical modulators.
According to paragraph 0019 in the publication, the electric-field-absorption-type optical modulation portions of the integrated optical modulation elements respectively have light absorption end wavelengths different from each other in the device according to the publication. Further, according to paragraph 0020 in the publication, the differences between the oscillation wavelengths and the light absorption end wavelengths are set within a certain range in the device according to the publication.
Other prior art includes Japanese Patent Laid-Open Publication No. 10-117040, Japanese Patent Laid-Open Publication No. 2001-144367, and Japanese Patent Laid-Open Publication No. 2001-326414.
As characteristic values of the electric-field-absorption-type optical modulator, the bandgap wavelength λBG of barrier layers and the photoluminescence wavelength λPL of light absorption layers in a case where a multi-quantum-well core layer is used as a light absorption layer exist. There are differences between the oscillation wavelengths that the plurality of laser element portions respectively have and the two characteristic values λBG and λPL that the plurality of electric-field-absorption-type optical modulators respectively have.
The inventors of the present invention earnestly studied and found that the differences between the oscillation wavelengths and the two characteristic values λBG and λPL influence characteristics, such as extinction ratio variation among the plurality of optical modulators.
As described above, the difference between the oscillation wavelength of the shortest-wavelength laser element and the oscillation wavelength of the longest-wavelength laser element has become larger in recent years. It is, therefore, necessary to attach importance to the differences between the oscillation wavelengths and the two characteristic values λBG and λPL that have not been taken into consideration. The inventors of the present invention has found that in an optical module having a plurality of integrated optical modulation elements, extinction ratio variation can be reduced by using new design parameters that have not been used.